The simplest way to get started with the amazing array of Web 2.0 tools is with a blog and or wiki.
There are many place you can go to create a blog, however, not all of them are accessible at school. Sites like Blogger can be blocked.
Some of the "school friendly" sites include

Blogging

A bit about blogging from the Redback blog

Edublogs

Globalteacher

Wikis

A bit about wikis from the Redback Blog

Wikispaces

PBwiki

Wetpaint